Deep frying oils and shortenings include those edible oils derived from corn, soybeans, palm, tallow and other vegetable and animal sources. Because of concerns over the harmful effects of cholesterol, however, nearly all fast food restaurants and food processors have converted to 100% vegetable oil for deep frying.
Unfortunately, vegetable oil (as a result of its unsaturated lipid structure) decomposes more readily than tallow when exposed to air and moisture at frying temperatures. Repeated food frying cycles subject vegetable oil to temperature swings and aeration which quickly lead to a runaway decomposition profile, resulting in a severely limited frying lifetime.
"Spent" frying oil is generally discarded when an unacceptable deterioration in food quality is experienced. Currently, disposal of "spent" vegetable oil is conducted approximately weekly. Institutions which claim no oil is discarded or recycled because most of he oil is lost to food absorption are faced with a mew challenge. Diet consciousness has created a market for coatings for fried foods to reduce oil absorption into the food product. The end result is less absorptive oil-loss and less necessary oil replenishment or "top-up," thereby increasing the overall content of decomposition products quicker than would be experienced with normal top-up.
Recent economic and regulatory changes in the frying industry dictate more prudent use of deep frying oil than currently practiced. Colorimetric tests such as Oxifrit.RTM. and Fritest.RTM. available from E. Merck of Darmstadt, Germany, make the polymeric content of frying oils easily measurable at both frying and room temperature. An additional test, VERI-FRY TPM Quick Test.RTM. available from Libra Laboratories of Piscataway, Ne Jersey, measures polar content with greater facility than either Oxifrit or Fritest. Upon the basis of polar (polymer, fatty acids and other oxidized products) content, governmental regulations may soon require more frequent disposal of used oil.
Indeed, several European Economic Community member states have recently adopted regulations setting forth maximum allowable polar content of frying oils. The maximum polar content established by these regulations, ranging between 16 and 27 percent, can be reached in two to three days of restaurant use. More frequent disposal brought about by government regulation will result in considerable financial loss to processors and/or users of frying oils.
Frying oil decomposition products generally arise in three ways: 1) hydrolysis; 2) solubilization; and 3) oxidation. Numerous systems have been developed to remove various decomposition products resulting from hydrolysis and solubilization. The minor polar contaminants arising from hydrolysis, namely fatty acids and fatty alcohols, are claimed to be efficiently removed by the technologies of silica and alumina adsorbency.
Undesirable color changes arising from solubilization as well as dissolved odors are claimed to be removed efficiently by periodic filtration and powdered activated carbon ("PAC"). U.S. Pat. Nos. 4,988,440 and 4,959,144 for example, disclose filter pads containing 15-40% silicates with 15-35% "Darco S-51," a PAC available from American Norit Company, Inc., of Jacksonville, Florida, or "Cecarbon PAC 200," available from Atochem of Pryor, Oklahoma, for use in frying preparations. Periodic filtration is also disclosed in U.S. Pat. No. 4,363,823.
Periodic filtration systems of various types are used to remove contaminants and decomposition products from used frying oil. U.S. Pat. No. 4,974,501, for example, discloses a deep fat frying apparatus in which the oil is periodically drained from a selected fry pot and filtered to remove particulate matter.
U.K. Patent No. 2 146 547 A discloses a cleaning container for edible oils removed from cooking vats. An activated carbon filter element is disclosed which operates to remove from edible oils gases and liquids which may cause smells.
JP 01,123,612 discloses a filter of pulp, PAC and clay molded and dried for periodic filtration of used cooking oil to remove offensive odors of fish.
JP 03,193,101, JP 03,193,102 and JP 03,193,103 disclose periodic filtration of spent edible oil from frying foods using a container made of fibrous material and having granular activated carbon packed therein.
JP 60,135,483 discloses a precoat technology of PAC and Celite.RTM. to clense oil.
Additional systems have been developed for continuously removing contaminants from edible oil. U.S. Pat. Nos. 4,957,758 and 4,962,698 disclose a method and apparatus for refining cooking oils in which the oil is continuously fed through a filter system immediately after starting the frying process and within the time free fatty acids or fatty acid oxidation products are formed. Filters of acrylic fibers are disclosed to remove food particulate matter (cracklings).
U.S. Pat. No. 5,008,122 discloses a process and apparatus for continuously removing contaminants from edible cooking oil by solvent extraction. A continuous stream of cooking oil at an elevated temperature is withdrawn from the cooking bath. A continuous stream of liquid solvent for at least a portion of the contaminants within the oil is then mixed with the oil. The oil is subsequently separated and returned to the cooking bath.
U.S. Pat. Nos. 4,487,691 and 4,668,390 disclose deep fat fryers with continuous filtration of frying oil through a helical- or serpentine-shaped, finned-tube heat exchanger followed by a filter comprising a carbon impregnated cloth. The filter housing is positioned to permit ready replacement without fat loss or tank draining.
U.S. Pat. No. 4,623,544 discloses an apparatus for deep fryers including a bypass-type oil filtering structure. The filtering structure, changed daily, removes food particulate matter (cracklings) in a fryer with a 90% efficient heat exchanger.
U.S. Pat. No. 4,704,290 discloses a recirculating-type deep fryer including filtration by means of an in-tank crumb tray to remove food particulate matter.
U.S. Pat. No. 3,977,973 discloses an integral, continuous filter to remove particulate matter with a removable frame.
At present, few if any commercially effective systems exist for reducing polymeric material content resulting from oxidafire decomposition. Oxidative decomposition results predominantly from the formation of hydroperoxides. Peroxide value ("PV") or buildup of peroxide content is directly proportional to the formation of quality-depleting carbonyl content or polymeric materials. Peroxide presence prior to decomposition is dependent on the temperature of the frying medium. At temperatures of 65.degree. C. and below, for example, formation is easily measurable and decomposition is very slow. Above 65.degree. C. to 110.degree. C., rate of formation is nearly equal to rate of decomposition. Above 110.degree. C., the peroxides decompose rapidly to a myriad of products whose selective adsorption would be difficult to control.
It is believed that control of peroxide value in edible oils may also control polymer formation. This control may be effected by antioxidants and chelation agents which interrupt the process of oxidation in oils.
U.S. Pat. No. 4,968,518 discloses a process for the treatment of frying or cooking oil comprising contacting at least a portion of used cooking oil with an aqueous solution containing one or more antioxidants. The antioxidants are selected from the group consisting of ethylenediaminetetraacetic acid ("EDTA"), n-propyl-3,4,5-trihydroxybenzoate ("propyl gallate") and mono-tertbutylhydroquinone ("TBHQ"). The cooking oil is first withdrawn from the cooker and cooled to a temperature below the boiling point of the aqueous solution by indirect heat transfer. The cooled cooking oil is then passed through the aqueous solution of antioxidants in an extraction vessel. After extraction, the oil is phase separated. The recycled oil may optionally be passed through an absorption zone including a suitable adsorbent, such as activated carbon to remove soluble oxidized and pyrolyzed contaminants from the oil.
Antioxidants, in general, have been developed primarily to lengthen shelf life of edible oils and have poor stability at high temperatures. Moreover, antioxidants are lost by steam distillation out of the oil or by absorption into the frying food.
U.S. Pat. No. 5,068,115 discloses a method for cleaning edible oils using an immersible packet containing the ground rind from a fruit, such as granulated grapefruit peelings. An antioxidant is provided by and carried with the ground rind of the fruit (the rind contains citric acid and ascobic acids which are known antioxidants). The method comprises immersion of a porous or perforated packet containing citrus peels into the edible oil. The porous packet is retained in the edible oil for at least five minutes during which time the edible oil can flow through the porous packet and contact the citrus peelings within the packet. At the end of the appropriate period of time, the porous packet is removed from the edible oil. It is also disclosed to prefilter the oil through a cellulosic filter impregnated with ground spices such as peppercorns or cloves.
A study on the effects of low-temperature, periodic filtration through several activated carbons on peroxide value, thiobarbituric acid and carbonyl values of autoxodized soybean oil is discussed by Boki, et al., J. Amer. Oil Chem. Soc. 68(8), 561-565 (1991). Substantial peroxide reduction at 60.degree. C. was found using 23 selected powdered activated carbons and granular activated carbons for edible oil storage studies. The results of the storage studies indicated that the quality of soybean oil treated with activated carbon was slightly better than that of an untreated control up to approximately 75 days, at which point oxidation was found to occur more rapidly in the treated oil. Decrease in the oxidative stability of the treated oil overtime was thought to result from removal of .alpha., .beta. and .gamma.-tocopherols.
The usefulness of activated carbons in filtration processes is limited according to at least one commentator because of the removal of the antioxidant tocopherol. Jacobson, G. A., Quality Control in Deep-Fat Frying Operations," Food Technology, 72 (February 1991).
U.S. Pat. No. 4,125,482 discloses an MgO impregnated activated carbon for use in the refining of edible vegetable oil. A previously degummed vegetable oil is passed through a bed of granular activated carbon impregnated from about 1.0% to about 15.0% by weight of MgO. The treatment is claimed to reduce the content of free fatty acids, phospholipids, peroxides and other impurities to improve the stability of the oil over its shelf life.
JP 58,020,152 discloses an antioxidant material comprised of zeolite, AC, alumina, activated white clay, silica or ion exchange resin to adsorb oxidation decomposed products and slow the oxidation. The method of treatment is by addition of the antioxidant material to the oil or by periodic filtration of the oil through the material.
McNeill, et al., J. Amer. Oil Chem. Soc. 63 (12), 1564-1567 (1986) discuss improvement of the quality of used frying oils by treatments with a combination of activated carbon and silica. Three levels of activated carbon (3%, 6% and 9% wt/wt) were blended with three levels of a silica compound (2%, 4% and 6% wt/wt) in all nine possible combinations in the study at 60.degree. C. For each treatment combination of silica and carbon, the average percent decrease for acid value, peroxide value, photometric color, polar compounds, saturated carbonyls and unsaturated carbonyls was studied. It was postulated that some type of flow-through cartridge system might be more efficient than the batch filtering system studied and might allow a greater throughput while minimizing oil losses.
Activated carbon, silica, Celite (diatomaceous earth) and alumina (a synergistic treatment) filtration systems thus have a presence in edible oil filtering process but an improved filtration process would be welcomed by the user.
A process which offers improved reduction of total polar content would be a seminal contribution to the frying industry.